The ATPase activities of the thymocyte is being studied in AKR mice with a high incidence of leukemia and mice strains with a low incidence of leukemia. The ATPase activity has been reported to be elevated in the AKR mouse. A very active but highly labile activity in thymocytes of AKR mice was observed which becomes increasingly active with age and the onset of leukemia. Studies are in progress to stabilize this activity which would allow its subsequent characterization. A study of the relation of this activity to other parameters of the preleukemic state will be made as well as studies of the effect of chemotherapeutic treatments on the activity. The relation of this activity to the maturation of the thymocyte and T-lymphocyte is also to be investigated.